I think it is probably because the in built flash is designed to only be able to cover to the widest 17mm, as you are using a 10mm lens the flash isnt high enough up to not cause a shadow from the lens, if you want to remedy this the only way to do it would be to buy an external flash, such as the 430 EX, this may not help at this wide angle, (i dont know I dont own ether) but thats how it might work. Even my 17-85mm on my 60d causes shadowing, so at most focal lengths if im trying to shoot somthing close I have to take of the lens hood, so if your using a hood try that, it might make the impact of the shadow less.

Gareth is right: at this wide angle you will get the shadow of your own lens on the picture when using the internal flash. Happens quite quickly, and there is nothing you can do about that. Just zoom in a bit more, or don't use the flash. No other alternative I am afraid...

In order to better explain what is going on, lets try some basic geometry.

So, with your 10-22mm on your camera, set it to 10mm.Now, have your flash up.Once that is done, with most of your hand resting on the bottom of the lens, stick one finger in front of the lens.Position your finger so that it is in focus, close to the lens, and towards the bottom.Now, turn your camera around (keeping your hand and finger in place in relation to the lens) and look at what is going on.You will see that your flash cannot get a line of sight to your finger as it is being blocked by the lens.Without this line of sight, it cannot light up the area.

Not only does such a large lens block the pop-up flash, which casts a shadow, this lens, as does all ultra-wide angle lenses, is more likely to have your shadow in the image when you are not using flash.

External flash is a wonderful thing, whether mounted on the camera's hot shoe, or even better, positioned away from the camera.